Regulation of metal ion recognition by allosteric effects in thiacalix[4]crown based receptors

Three new ditopic receptors 3a–c based on thiacalix[4]arene of 1,3 alternate conformation possessing two different complexation sites have been designed and synthesized for both soft and hard metal ions. The imino nitrogens bind soft metal ion (Ag+/Pb2+/Cu2+) and the crown moiety binds K+ ion. The preliminary investigations show that 3a–c behave as ditopic receptors for Ag+/K+, Pb2+/K+, and Cu2+/K+ ions, respectively. In all the three receptors it was observed that the formation of 3a·Ag+/3b·Pb2+/3c·Cu2+ complex triggers the decomplexation of K+ ion from crown moiety and acts as a gateway, which regulates the binding of alkali metal to crown moiety. Thus, allosteric binding between metal ions ‘switch off’ the recognition ability of crown ether ring.